Adolfo: I still would like full certainty about FCP's 10-bit internal processing.

Vegas has supported 10-bit output since many years, and uncompressed at least since Blackmagic released their 10-bit Windows codec. Unfortunately, Vegas will internally truncate the output of any effects or transitions to 8-bit. These must be handled in Combustion or equivalent and put back on the timeline in 10-bit, after which Vegas can render to any supported 10-bit codec without loss.

My concern is that having a 10-bit codec doesn't guarantee that the internal effects and transitions output true 10-bit. It didn't in Vegas.

I like the extra latitude in post that 10-bit gives me, even as the eventual rendering will be in 8-bit. It's not that it gives me a "better picture" as much as it is giving me more freedom in color correction and other post processing.